1. Field of the Invention
The present invention relates to a magnetic recording medium having a coat-type magnetic layer, and more specifically to a magnetic recording medium with errors improved in the MR head recording-reproducing system.
2. Disclosure of the Related Art
In recent years, the magnetic recording medium such as VTR tape, audio tape, computer tape, etc. is required for high-density recording for increased performance, recording longer time, and reduced size and weight, and a system that uses the MR head is presently under investigation.
Because when the MR head is used, high-hardness AlTiC is frequently used for the MR head sliding portion, with the coating layer design such as the conventional magnetic recording medium, problems occur in durability.
For cases with a combination with the MR head reproducing system taken into account, for example, in Japanese Laid-open Patent Publication No. Hei 10-312525 (1998), there is disclosed that the saturated magnetic flux density of the magnetic layer with the hexagonal ferrite powder is 300G to 1000G and the coercive force is 2000 Oe or more, or the saturated magnetic flux density of the magnetic layer with ferromagnetic metal powder is 800G to 1500G and the coercive force is 2000 Oe or more. In Japanese Laid-open Patent Publication No. Hei 10-302243 (1998), there is disclosed that the projection height on the magnetic layer surface and the reversal-volume of magnetization are prescribed and the coercive force of the magnetic layer is 2000 Oe or more.
However, there has not yet been obtained a magnetic recording medium that provides satisfactory durability suited for the MR head.
Accordingly, it is an object of the present invention to provide a high-density magnetic recording medium that can record and reproduce without any problem even if a head with an extremely hard sliding surface is used and that provides remarkable durability.
The inventor of the present invention worked assiduously and has found that even in an MR (magneto-resistive) head recording-reproducing system, a magnetic recording medium with excellent durability is able to be obtained by specifying the plastic deformation hardness of the sub-surface-layer of the magnetic layer and Young""s modulus in the thickness direction of the magnetic layer to a specific range, and has completed the present invention.
That is, the present invention relates to a magnetic recording medium comprising at least a lower non-magnetic layer containing at least a non-magnetic powder and a binder resin formed on a non-magnetic support and an upper magnetic layer formed on the lower non-magnetic layer, wherein the magnetic recording medium is used for a recording-reproducing system in which recorded signals are reproduced by an MR head, and a plastic deformation hardness Hv from the upper magnetic layer surface to an indentation depth of 0.1 xcexcm is 100 to 200 mgf/xcexcm2 and a Young""s modulus E in a thickness direction from the upper magnetic layer surface to the indentation depth of 0.1 xcexcm is 2000 to 3000 mgf/xcexcm2.
The binder resin contained in the lower non-magnetic layer is preferably radiation curing type binder resin.
The upper magnetic layer preferably contains a resin with polar functional group as a binder resin. A thickness of the upper magnetic layer is preferably 0.3 xcexcm or less.
According to the present invention, because the plastic deformation hardness Hv of the sub-surface-layer of the magnetic layer is specified to 100 to 200 mgf/xcexcm2 and the Young""s modulus E in the thickness direction to 2000 to 3000 mgf/xcexcm2, durability is able to be improved in a system using the MR head. Now, the plastic deformation hardness Hv of the sub-surface-layer and the Young""s modulus E in the thickness direction mean respectively the plastic deformation hardness Hv and the Young""s modulus E in the thickness direction from the surface of the upper magnetic layer of the medium to the indentation depth of 0.1 xcexcm.
When the plastic deformation hardness Hv (plastic hardness) and/or the Young""s modulus E in the thickness direction (elastic hardness) deviate from the scope of the present invention, nonconformity as shown below occurs. When the plastic deformation hardness is smaller than 100 mgf/xcexcm2 and/or when the Young""s modulus in the thickness direction is smaller than 2000 mgf/xcexcm2, the coating layer is scraped at the head sliding section and durability is degraded. When the plastic deformation hardness is greater than 200 mgf/xcexcm2, the easiness of slitting is degraded and error is aggravated because of omission of coating layer from the tape edge portion. When the Young""s modulus in the thickness direction is greater than 3000 mgf/xcexcm2, spacing loss is generated with the head because of excessively hard coating layer, and the lowered output results. In the present invention, the preferable plastic deformation hardness Hv is in a range of 120 to 170 mgf/xcexcm2, and the preferable Young""s modulus E in the thickness direction is in a range of 2300 to 2750 mgf/xcexcm2. It is more suitable if both Hv and E are respectively in the preferable ranges.
There are several methods for controlling respectively the plastic deformation hardness and the Young""s modulus in the thickness direction to the above specific ranges. The present invention is not particularly limited to the following, but for example, methods such as adjusting the elastic modulus by combining a plurality of binders in the magnetic layer, adjusting the dispersibility of the coating layer by the binder resin with polar functional group introduced, or adjusting calendaring conditions or thermosetting conditions are effective. In particular, the plastic deformation hardness Hv tends to increase by increasing the calendaring conditions (still higher temperature, higher linear pressure). The Young""s module E in the thickness direction tends to increase by increasing the thermosetting temperature. Consequently, even when the same magnetic coating material is used, it is recommended to investigate various conditions for preparing the magnetic recording medium as required.
According to the present invention, a high-density magnetic recording medium with remarkable durability that can records and reproduces easily even when the MR head with an extremely hard sliding surface is used.